PHP is an HTML-embedded scripting language. Much of
its syntax is borrowed from C, Java and Perl with a couple of unique
PHP-specific features thrown in. The goal of the language is to
allow web developers to write dynamically generated pages quickly.

PHP/FI 2.0 is an early and no longer supported version of PHP. PHP 3
is the successor to PHP/FI 2.0 and is a lot nicer. PHP 5 is the current
generation of PHP, which uses the » Zend
engine 2 which, among other things, offers many additional
OOP features.

Can I run several versions of PHP at the same time?

Yes. See the INSTALL file that is included
in the PHP source distribution.

What are the differences between PHP 4 and PHP 5?

While PHP 5 was purposely designed to be as compatible as
possible with previous versions, there are some significant
changes. Some of these changes include:

You should go to the PHP Bug Database and make sure the bug
isn't a known bug. If you don't see it in the database, use
the reporting form to report the bug. It is important to use
the bug database instead of just sending an email to one of the
mailing lists because the bug will have a tracking number assigned
and it will then be possible for you to go back later and check
on the status of the bug. The bug database can be found at
» http://bugs.php.net/.